The Research Plan describes a series of experiments that will examine how spatial information is processed in the mammalian brain. In previous studies a population of neurons was identified within the mammillary nuclei -->anterior thalamus -->hippocampal formation axis that discharge as a function of the animal's head direction (HD), independent of the animal's behavior and spatial location. This spatial signal provides a model system for examining how primary sensory information, entering through various sensory pathways, is transformed into a "higher level cognitive signal" representing the organism's spatial relationship with its environment. The mechanisms that accomplish this transformation in the central nervous system are not known. The first aim contains 5 experiments and is designed to determine how the head direction signal is derived and processed from known sensory inputs. The question being asked is: how is primary sensory information, entering over various sensory pathways, transformed into a signal which represents the animal's directional orientation with respect to its environment? The second aim will better define the underlying anatomical connections within the HD cell system. The third aim will determine how visual landmark spatial information is processed in the brain. The fourth aim seeks to understand the functional significance of the HD signals to the organism;that is, how does an animal use these cells for orientation and navigation? In sum, these studies will provide insight into how spatial information is organized and processed in the brain and will enhance our understanding of the functional role of HD cells during navigation. The results will have implications for human health and behavior. It is common for elderly patients and patients with Alzheimers disease, a disease often associated with marked pathology in limbic system structures, to experience spatial disorientation to the extent that constant supervision is required. Learning how spatial information is processed in the rat brain will give us clues about the complex nature of spatial processes in humans.